Conventional data networks, such as hybrid fiber coaxial cable networks, optical fiber or coaxial cable networks, wireless networks, satellite networks, and the like, allow one or more devices to communicate with one another, often times sharing access to a common transmission medium. In the example network depicted in FIG. 1, a central office 100 (e.g., a content provider, headend, etc.) may be connected to a distribution network over nodes 101a-b. Various homes 102a-f may be connected to the network, and may receive signals carrying content from the central office 100. The content may be, for example, television programming and movies received from content providers, stored at the central office 100 or received from another source using an external network (e.g., an Internet Protocol backbone network, a satellite network, etc.). The signal loss over fiber optic cable is much less than the loss over coaxial cable, thus the fiber optic connection between the central office and the fiber node 101A is often very long (e.g., 10 to 40 km). The signals over coaxial cable are much easier to split and amplify and terminate than fiber optic cable, thus the final connection from the fiber node 101A to the customers home devices is typically done with coaxial cable.
The central office 100 may offer downstream signals carrying this information (as illustrated by the arrows in FIG. 1). The central office 100 may also receive upstream communications from the various homes 102a-f. For example, in one type of system under the Data Over Cable System Interface Specification standard (DOCSIS), the available bandwidth on the distribution network is divided by frequency range into a downstream portion and an upstream portion. Most of the frequency allocation is in the downstream direction (e.g., 54-860 MHz), while a smaller portion (e.g., 5-42 MHz) is allocated for upstream transmissions. Since the various homes 102a-f need to share the transmission medium for their upstream transmissions, the DOCSIS standard calls for that sharing to be controlled by a termination system, such as a Cable Modem Termination System (CMTS) at the central office 100. Under DOCSIS, the CMTS instructs the various homes 102a-f (or, the cable modems in those homes) on when they can use the upstream bandwidth.
The centralized approach in DOCSIS allows for the centralized management of the upstream bandwidth, but it has a disadvantage. Upstream transmissions to a given CMTS are carried on a selected frequency channel within the 5-42 MHz upstream allocation, but the entire 5-42 MHz range is not used by all homes, because some homes suffer interference within that range. For example, home 102a may be located near a source of electromagnetic interference in that range, while home 102f may need to traverse a longer geographic distance, and pass through more splitters 101a-c, to reach the CMTS (resulting in greater loss of signal in different ranges), suffering interference along the way (particularly at the lower ends of the upstream range). Other homes, however, might not suffer these drawbacks. Some homes 102d might be geographically closer to the central office 100 and its CMTS, some 102b-c may have fewer intervening splitter nodes, and others 102e may have an intervening fiber optic portion 104 that allows signals to avoid much of the interference that would otherwise be suffered along the path of the fiber optic cable. Some of these homes may be able to use other frequency portions that are not within a chosen DOCSIS upstream channel.
There is an ever-present need to offer greater data transmission capability to end users.
Furthermore, the upstream channel may be nonlinear, and that may present issues for transmission. Most other media for communication are linear, such as twisted pair, satellite, and wireless. In hybrid fiber coaxial (HFC) networks, for example, the conversion from coaxial cable to fiber optic cable in the upstream requires an optical laser transmitter to be placed in the transmission path between modem transmitter and modem termination system receiver. This leads to nonlinear distortion in the HFC upstream channel that is not present in most communications systems. Also, since the upstream spectrum allocation is so low in frequency, 5-42 MHz, there is very little signal attenuation in the upstream. Most communication systems have a flat noise response over the communications channel such as wireless systems or a noise response that gets worse at higher frequencies such as DSL. In these systems the primary noise determinant is the signal attenuation and the receiver noise floor. In the HFC upstream, the noise is worse at lower portions of the spectrum since the noise level tends to decrease with higher frequency but the signal level stays relatively flat. Since the upstream is amplified often in the HFC upstream path and the fiber optic link has very little attenuation, the noise floor is not determined by signal attenuation and receiver noise floor. In the HFC return path channel, the signal-to-noise ratio tends to improve as the frequency increases and is fundamentally determined by external noise sources, upstream modem transmit power limitations, return path amplifiers and filters, and the noise and distortion characteristics of the optical return path laser transmitter in the fiber node.